text:
Where can I find canonical examples of radio communications at non-towered airports?
I'm trying to find any canonical examples of radio communication patterns at non-towered airports. The FAR/AIM has very little in terms of sample announcements, etc. The AC-90-66B (https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_90-66B.pdf) doesn't offer much either. The best resource I could find so far is the "Say Again, Please" book by Bob Gardner, but I'm trying to find the guidelines from the "FAA's mouth."

Is AIM 4-1-9(g)(6) what you’re looking for?

avoid saying "taking the active" since there really is no defined "active" runway, despite what traffic may be currently using or wind may be currently favoring...

@StephenS, I'll take that as an answer. I read through that shortly before posting the question but somehow ended up thinking "there ought to be more on that," LOL. Seems to me, the subject warrants a decent Advisory Circular.

What you're looking for is AIM 4-1-9(g)(6), recommended self-announce phraseologies. The self-announce phrases states position, altitude, climb/descent, among other things depending on the situation.

6. Recommended self-announce phraseologies:
It should be noted that aircraft operating to or from
another nearby airport may be making self-announce
broadcasts on the same UNICOM or MULTICOM
frequency. To help identify one airport from another,
the airport name should be spoken at the beginning
and end of each self-announce transmission.
(a) Inbound
EXAMPLE−
Strawn traffic, Apache Two Two Five Zulu, (position),
(altitude), (descending) or entering downwind/base/final
(as appropriate) runway one seven full stop, touch−and−
go, Strawn.
Strawn traffic Apache Two Two Five Zulu clear of runway
one seven Strawn.
(b) Outbound
EXAMPLE−
Strawn traffic, Queen Air Seven One Five Five Bravo
(location on airport) taxiing to runway two six Strawn.
Strawn traffic, Queen Air Seven One Five Five Bravo
departing runway two six. Departing the pattern to the
(direction), climbing to (altitude) Strawn.
(c) Practice Instrument Approach
EXAMPLE−
Strawn traffic, Cessna Two One Four Three Quebec
(position from airport) inbound descending through
(altitude) practice (name of approach) approach runway
three five Strawn.
Strawn traffic, Cessna Two One Four Three Quebec
practice (type) approach completed or terminated runway
three five Strawn.

Examples shown are from the Aeronautical Information Manual. I hope that helps!


text:
Do helicopters have trim? How does it work?
I recall having read somewhere that “helicopter in forward flight behaves just like a badly trimmed airplane”. So I'd like to ask:

Do helicopters have trim?
If so, how does it work?
And is the helicopter stable when properly trimmed?

It depends on what you mean by trim.
If you mean a a mechanism that allows the pilot to reduce the control forces in order to allow the helicopter to be flown hands off, then the answer is a qualified yes.
If you mean a trim tab attached to a control surface, then the answer is no. Many airplanes which have a trim control do not use a trim tab. For example, Pipers have an anti-servo tab on the elevator, which the pilot uses to reduce control forces.

Since helicopters are unstable -- meaning if you take your hand off the cyclic the aircraft will be in an unusual attitude within a second -- nothing short of electronics and actuators will allow a heli to be flown hands off.
But every heli except the smallest ones, have considerable stick forces, which make them difficult to impossible to handle without some kind of assistance.
The simplest setup is what you find on the original 4-seat R-44 Astro, which lacked hydraulic assist. On top of the cyclic you will find a 4-way "hat" switch which uses electrical actuators on the cyclic to reduce the control forces necessary to fly it.

Larger helicopters have hydraulic assist, which allows the copter to be controlled with just two fingers. Inop hydraulics is considered an emergency.

Big helicopters have a stability augmentation system, which allows the helicopter to be flown hands off. An SAS is required for IFR flight.

The smallest ones, e.g. R22, have collective and cyclic friction which can be used, but are usually not used, to relieve force on the controls.  In all POIs I have seen, frictions are to confirmed off for takeoff and there is no guidance for their use in flight.  I certainly would not try to use them, although I have no doubt if it all went quiet, I would be able to get that collective to the floor regardless of friction :)

I don't use cyclic friction on the R22

Nope, although for some pilots, limited cyclic authority in most flight phases might be a life saver ;).  I'm thinking about pulling back in -ve G etc.

Many helicopters have stabilizers on the tail similar to aircraft. Don't these add some stability in forward flight?

Not enough to overcome the inherent instability. The stabs are very useful if you lose the tail rotor

it looks like the Robinson R-66 is getting an autopilot http://www.heliopsmag.com/news/new-r66-options

Stanley Hiller's "Rotor-Matic" cyclic control system, with two small servo rotor paddles offset 90 degrees to the main rotor blades instead of the flyweights seen on the early Bell Model 47 helicopter, despite the lack of electronic and hydraulic stability systems, was capable of a stable "hands-off" hover.  It was, however, a purely mechanical servo, not a "trim" system.

Helicopters with hydraulic actuation of the swash plate usually have an artificial feel spring. The neutral position (where the stick returns to if released) of this feel spring is the trim position. The pilot holds the cyclic at the position where the helicopter has a steady flight path, and can then trim the feel force away by adjusting trim position in two possible ways:

Beep trim. A 4-way china hat switch at the control is deflected by the pilot, resulting in slow continuous adjustment of the neutral spring position. With a stick held in position, the feel spring forces are gradually reduced until the pilot stops deflecting the switch when feel force is zero.
Trim release. A push button that effectively removes all feel spring force: neutral spring position always follows stick position. With trim release ON the stick can easily fall over if friction is low. Pilot places the stick at steady flight path, then switches trim release OFF. The feel spring is now effective again.

Is the helicopter stable when properly trimmed. We need two answers here, for forward flight and for hover.

In forward flight: Directionally, yes. Pitch - often, if the heli has a horizontal tail. Depends on position of horizontal tail relative to rotor downwash, which varies with airspeed and gross weight. Roll - usually not. That's considering aerodynamic stability only, an AFCS with heading & altitude hold would make for a docile trim situation.
In hover: it's all unstable everywhere, like trying to keep your balance when standing on top of a large inflatable ball. "Trimmed" has very little meaning in the hover.

The R-22 helicopter has both cyclic/collective friction, as well as a trim. When you are hovering, you are using a lot of tail rotor to counteract the spinning of the blade, and there isn't any force to counteract it. The tail rotor, while countering your rotation, will also push you in a direction (like a propeller), so in a hover, with a helicopter that has a counter clockwise rotation, the tail rotor will keep pushing you to the right. To counter that, you have to put in left cyclic. To help reduce the effort required to keep it a hover, Robinson put in a trim, which just adds a constant left input to the cyclic. This greatly reduces the workload for the pilot while taxiing and hovering. The amount of trim, in the case of Robinson helicopters, is not pilot adjustable, but it is a trim setting, technically speaking, which is separate from the friction settings, which helicopter pilots can/do use in the same way fixed wing pilots would use a "trim". For example, I've used the cyclic/collective friction settings in the R-22 to tighten up the movements of the cyclic and collective. This makes it much easier to read a navigation map or use the radio. It allows me to take my hands off some of the controls for a whole second or two to allow me to adjust a map or tune the radio :)

Yes. Some will use a "trim" function to hold the cyclic or collective stick in place to prevent rapid changes in the blades pitch. Some blades also have "trim tabs" and are critical component on the blade and require adjustment to ensure the smoothest flight.

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text:
Which wheel or wheels get the chocks?
As a quick follow-up question to my earlier one about the retail cost of chocks, which wheel(s) typically get the chocks on a GA tricycle-gear plane? Do you buy one roped pair for the nose wheel (make sure your chocks are away before starting up), or two pair for the main gear, or all three (I doubt this, especially on a small plane)?

To be most effective chocks should be placed on both of the main gear of light aircraft. This prevents the aircraft from rotating or pivoting about the chocked wheel.There's a nice article from the Flight Safety Foundation which talks about this.
The FAA, in an overabundance of caution, advises you to chock ALL the wheels (see AC 20-35C).

Practically speaking, on a flat ramp and a calm day chocking any single wheel is usually enough to keep the airplane where you put it for a short while (say long enough refuel the aircraft, or to get lunch).
When using a single chock my preference in these cases is to chock the nose wheel as it's the most visible (I fly a low-wing airplane, and an inattentive line person who needs to tow the plane could miss a chock on the main gear). With two chocks the main gear should be secured on each side of the aircraft to keep it from pivoting.
For anything longer than that a brief food and fuel stop, or when high winds are anticipated, aircraft should be properly secured (by tying them down and chocking at least one wheel).
In these cases the tiedowns serve as the primary securing mechanism, and the chocks as a secondary mechanism (keeping the plane from rocking fore and aft in high winds and breaking the tiedown ropes).

Note that there is an exception to the "high winds" bit here: If winds strong enough to blow chocks around on the ramp are anticipated, such as a hurricane, the chocks should be removed unless they can be adequately secured to ensure they don't get blown into an aircraft.
  Of course if winds that strong are anticipated you should really have flown away before they got to you, or you should be looking for a hangar to weather the storm in.

Normally, the chocks are put on the main wheels (the wheels under the wings).
If you just chock the nose wheel and there is no tie down then the aircraft could pivot/rotate.
Just to clarify the situation... At airports there are two main parking areas, the "ramp" and the parking area. Usually there are tiedowns only in the parking area. Also, the parking area is allocated, so you have to get permission to take up a slot there. When you visit an airport short term most crews therefore park on the ramp and chock the wheels. The problem arises that "short term" might turn into hours or overnight. If strong winds develop, then a poorly chocked aircraft can pivot, come loose and roll away.

What if gust locks/control column locks were also used, preventing the nose wheel turning? I ask because in the related question about cost, one of the pictures shows a nose wheel chock.

@KeithS The nose strut/steering linkage isn't usually what pivots in these cases - the contact patch of the chocked tire scrubs against the ground as the aircraft rotates around it. This can (and does) happen when just one main tire is chocked too, which is why the best thing to do is chock the main gear on both side of the aircraft (or ideally all of the wheels) so the plane can't pivot at all.

OIC. Good answer.

@KeithS If you chock the aircraft, you may as well do it right. The kind of pilot that just puts a block on the nose wheel then drives off to the square dance is the kind of pilot that ends up with $10,000 repair bills after his plane rolls into some corporate Gulfstream.

For tailwheel aircraft, I chock the main on the side with the door (usually right), and the tail to keep it from pivoting. If there's a tail tie down,  I will always tie down the tailwheel, and I will attempt to park into the wind.
As my instructor used to say "the flight's not over till the tailwheel is tied down"

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text:
Regarding N1 and N2, what does the "N" stand for?
I initially phrased this question in a comment to another question regarding what N1 and N2 mean, hoping it would be that sort of low-hanging fruit that is easily answerable and not worth a proper question in its own right.  But then I starting searching and came up short.
This Yahoo Answers page has lots of guesses on it, from "nominal" to "Newell".  (And we all now are reminded why nobody goes to that site for real answers.)  Many other pages explain what N1 and N2 are, but not where the "N" part comes from.
I'm wondering if it might have its origin in German, as a lot of early developmental work on jet engines was done in Germany.  Or was it simply an unused letter that fit?

N stands for an unknown number, and since RPM is given as 1/min, the N is simply the number of revolutions per minute. Doing this is common in Germany; whether it originated in Germany is unknown to me.

As far as I can tell, the N comes from the physics behind turbines in general, whether they be water pumps or wind turbines, a subset of mathematics called fluid dynamics.  Many thanks to ROIMaison's answer for steering me in the right direction.
In Turbomachinery: Basic Theory and Applications, Second Edition, Earl Logan, Jr. provides a symbol reference list at the end of each chapter starting in chapter 2, defining $N$ as rotor speed, in units of rad/s; an angular change per unit time, and $N_s$ as the specific speed, a dimension-less quantity derived from a ratio of the flow and head coefficients.

N1 and N2 seem to be derived from both.  Wikipedia mentions the speed as a percentage paradigm as a human factors consideration, though as of 5/14/2015 there are no references to support this.  However, there may be a scientific basis behind this as well, since a change in N is proportional to the resultant change in airflow (i.e. thrust produced):

I would say $N_C = \frac{N}{\sqrt{\frac{T}{T_{ISA}}}}$ is proportional to thrust, not $N$ itself (which is why those values/percentages are shown to the pilot).

i'm not suggesting (for example) a 10% increase in N means 10% more thrust.  i'm saying that as one increases, so does the other.

Ok, if all other variables stay the same, that's certainly true - but that doesn't mean that they are proportional.
The great thing about $N_C$ is precisely that it is approximately proportional to thrust.

Here at Tech ops, they say it comes from specific speed.
However, it does not explain why the letter N was chosen, but perhaps this knowledge might help lead to an answer.
This source mentions:

N is the symbol for "specific speed". Specific speed is defined as "the speed of an ideal pump geometrically similar to the actual pump, which when running at this speed will raise a unit of volume, in a unit of time through a unit of head".

interesting from the mcnally source... i'd never thought of it from an airflow standpoint.

For rotational speed, $n$ is a common symbol. While the English Wikipedia article uses $\omega_{cyc}$, I can find many English-language sites using the symbol. To be honest, $\omega_{cyc}$ is a really combersome symbol for something that is used as often as rotational speed is.
In German (see the corresponding Wikipedia article), it is definitely the standard way of representing rotational speed in revolutions per time unit (as already mentioned by Peter Kämpf above). The origin of this is probably harder to determine, but I imagine that it has something to do with the number of revolutions always being a natural number. After all, $n$ is the go-to symbol for any unit-free integer value in engineering and physics, in my experience.
The usage of the capital letter, which is inconsistent with this, is a little more dubious in my opinion. But I have a theory for that, too: The majority of all software working with the value has, for the last 50 years, probably been written in Fortran. Fortran variable names are, by convention, capitalized. So anyone working with the value or displaying it might have just adopted the name of the variable as the symbol for N1 and N2.
Any pre-Fortran occurence of the terms might disprove that theory.

Fortran is more than 50 years old by now, and still used by Real Programmers!

Thanks, corrected the age. And yes, it's still used in many companies, if not in most, in the engine manufacturing industry.

Fortran by default isn't sensitive to case.

@mins: As described in the Fortran 77 standard, sections 3.1.1 and 3.2 (see https://www.fortran.com/F77_std/f77_std.html ), no lowercase letters are allowed in Fortran code with few exceptions.

@JulianHzg: Lowercase letters were usually converted into uppercase by the compiler. I think in F90 or F95, the compiler got an option to preserve the case to deal with external library names in C (for instance). Note that extensions to F77 existed to allow lowercase use in some cases, but the problem was also the punched cards didn't support lowercase characters.


text:
Could a light helicopter pilot land a transport-category helicopter?
Related question, but not a dupe as that's about starch-wings and the gap between light fixed-wing planes and fixed-wing transports does seem larger than the similar gap in rotorcraft...
Let's say our good friend Simon, with his considerable amount of cyclic-time in small-to-modest helicopters, is onboard an S-76 (or other similar large helo) and both the pilots find out that the fish was a bad choice of preflight meal.  How much assistance would he need from someone on the ground with time-in-type in order to get the helicopter on the ground safely, assuming this was in VMC?

This is all speculation, not sure this type of question fits the factual format needed... this is going to be opinion based at best.

I'd suggest that there are enough objective, factual statements that can be made regarding the differences & similarities between large/small helicoptors that there is ample room for non-opinion answers.

One might argue that the difference between the larges and common small helicopters is not as vast as that between the largest and small common airplanes. However hoe that plays into flight mechanics may vary.

This is based on knowledge and a little experience. I have flown a Wessex to a hover and they are big, heavy, powerful beasts. If your example

our good friend Simon

refers to me, then I must also state that I only have about 130 hours so limited hours is more accurate :)
It is said that if you can fly one helicopter, then you can fly another.  Indeed, I learned on the R22 and it's a "fact" (insofar as if enough people repeat something enough times, it becomes a "fact") that if you can fly the R22, then you can fly anything because it demands careful handling and has very small power margins.  I have never heard a really experienced pilot refute that.
If we assume that you have enough fuel and that there is no complex fuel system which requires switching of pumps to keep the engines fed, then yes, you could land anything with one notable exception.
Nearly all helicopters have a couple, either manual or automatic, between the demand for power (how much pitch you are asking for) and the engine power output.  Raise the collective and the throttle is automatically opened and vice-versa.  This is certainly true about transport category aircraft.  On something like early Hilliers, I would need to learn pretty fast that as I increase pitch, I must roll on throttle otherwise, I will stall when I try to climb or raise the lever to come into the hover.  This is practised during PPL but I must admit, I've never used manual throttle control since I got my license.
Another area of concern would be over controlling but that's easy to deal with.  I am now thrust into the pilots seat of a type I don't know and I am not licensed for.  I am going to land on the nearest safe spot.  I'm not going to do anything fancy whatsoever so, if I need a descent, I will lower the lever gently and use the cyclic to maintain my speed. Then I'll climb a little by gently raising the cyclic until I start to climb.  All helicopters fly nicely at about 60 kts so I will pitch for that speed. The only thing I need to look at to ensure the limits aren't busted is airspeed.  Any helicopter pilot would get the feel of this in very little time as would any fixed wing pilot.  It's just a question of how heavy the controls are and how much they physically need to move but that's quickly explored by starting gently and using just enough to do what you need to do.
Let's assume that the subject craft does have automatic throttle control.
I will also assume that we are in VMC otherwise, we are all going to die.
There are only two other areas of concern that I can think of and that's the type of the landing gear (skids vs wheels and how many) and the height of your seat above the ground.
One major difference between rotaries and fixed wing is that there is no stall speed and all helicopters can land with the same forward speed and rate of descent.  I can, if I want to, bring the craft to a hover and stop to have a good think about what comes next.  To come into a hover is also easy.  You can do it by the numbers but the practical way is to keep your speed over the ground so that the site picture is apparently a fast walking speed and keep the landing spot in the centre of the window.  if the landing spot moves up, decrease your rate of descent and vice-versa.  Do this, and you will arrive close to the spot at a sensible speed and with a low rate of descent.  Vortex ring state won't be a concern for me.  I won't know at which speed I will lose translational lift but it will be somewhere between 10 and 15 kt, so I'll be waiting for that little shudder to start and the nose to swing a little, ready to bring in power and pedal.
I believe that most, perhaps all, wheeled transport helicopters have wheel brakes, but I will have no clue how use these.  So, I am going to land zero-zero and ensure that I am directly into wind and on a level surface.  The next think is how will I know when I've landed?
When I first landed a B206, it was on my conversion course so I already knew how to land smoothly.  All I had to do to deal with the extra height I was sitting above the ground was "feel" the aircraft down.  It can be surprisingly difficult to know when a helicopter is fully on the ground and not flying it all the way down is one way to get into trouble.
I would deal with this by feeling for the first contact and then smoothly, but not too quickly, lowering the lever.  This does two things.  It ensures a firm contact and removes my biggest landing worry in an unfamiliar type which is dynamic rollover.  It won't be pretty and I might even get a blade strike somewhere but with power now off, we'll all walk away.
So in summary, I wouldn't need any assistance but, if I can figure out where the transmit button is and how to tune the radio, I will try to get some anyway.

So many relevant details in an easy reading narrative form. Good idea!

Now I want Simon to take me for a ride!

What if you had to save the day in a Chinook? Does a tandem rotor craft work with the same basic set of principals?

@TomMcW Hi Tom.  Simply put, yes.  There are no fundamental differences. It will just need less pedal work since there is no torque reaction to pedal changes.

@Simon I'm going to have to read up on tandem rotors. After reading in SE I've finally got a general idea of how helicopters fly, I just can't picture the flight dynamics of a Chinook in my head like a standard helo.

@TomMcW Anything specific or just in general?

@Simon Just in general. If I get a chance I might post a question or two. I don't think we have many tandem rotor questions on here. Surprising since the CH-46/47 are such successful aircraft.

@TomMcW Cool, you should post questions.  I agree they would be good to have on the site.  In essence, tandem rotors remove the need to use some engine power (which can be significant) to drive a tail rotor, since the torque reaction of the rotors cancel each other, meaning more power for creating lift.  They also have a much wider longitudanal range for the CofG envelope making them great for shifting cargo.


text:
How can a sUAS operator contact ATC/identify themselves when contacting ATC?
In a comment, abelenky offered some words of wisdom which I personally agree with regarding sUAS ops near a surface Class C:

When in doubt or close, call Tower and state your intentions. They'll approve you, and if you cross into their airspace you will be legal. If you stay outside their airspace, you asked approval for no reason, but no harm done.

However, how should a sUAS operator contact an ATC entity that has jurisdiction over surface airspace (such as Tucson or Davis-Monathan Tower, in the querent's example)?  Should they place a phone call to the Tower phone number? Call the Tower on frequency with a handheld airband radio?
Further, is it even legal for a sUAS operator to use an airband radio in the US (i.e. does a commercial sUAS certificate count as a FCC aeronautical-mobile license the same way a regular PPL/... does)?  And if they were to call the Tower on the radio, how should they ID themselves/what callsign should they use, as I am not sure if a sUAS registration number is a valid radiotelephone callsign?

"does a commercial sUAS certificate count as a FCC aeronautical-mobile license the same way a regular PPL/... does" - Might be worth noting: by my reading of the FCC rules, a PPL doesn't count as an FCC license. The rule states that a license is not needed to use an airband radio as long as that radio is located on board an aircraft. So PPL holders are not permitted to transmit while not on board an aircraft (unless they have a radio license), and non-PPL occupants of an aircraft are allowed to transmit.

This question may well be obsolete due to recent changes in regulations, and the introduction of the LAANC authorization system.

First, you should NOT transmit on air bands as a UAS pilot.  You do not have a ground station license and it would be a violation of both FCC and FAA rules, contact with the tower, when necessary, should be done via the phone.  It is fine (and I'd even encourage) to monitor the air bands relevant to the location you are flying in to be aware of on-going traffic.
How to approach contacting the tower depends on what you are flying your sUAS as.  LAANC (Low Altitude Authorization and Notification Capability) is now active almost everywhere, though some particular airports still do not use it.  LAANC should be used whenever available, however the traditional approach will still need to be used for any particular airports not yet participating in LAANC.
LAANC
If you are flying near an airport where LAANC is available, then you can simply use an app from your mobile phone to request flight authorization/notification.  You simply enter the area or flight plan you will be flying and hit the button to request approval and then wait for the notification it's granted.
This can be done regardless of if you are a part 107 pilot or a recreational pilot.  As long as you are flying below the maximum altitudes defined in the UAS Facilities Maps, you should receive near immediate approval in most cases.  If you are flying above these limits, you will need to be a 107 pilot and go through the traditional waiver process.
You can find a listing of apps that currently support LAANC listed on the FAA's website here.
Traditional Notification/Waiver
If the airport you are flying near does not support LAANC, then if you are flying for purely recreational purposes, you must provide notification to the airport operator and ATC prior to your flight.  You should use the telephone contact information to do this.  As long as they do not specifically deny your flight, you only have to provide notification.
If you are flying under part 107 (FAA certified remote pilot), you will need either an airspace authorization (easier to get and may be requested up to 6 months) or an airspace waiver (harder to get and may be requested for up to 2 years).  You should check the UAS Facilities Maps to find the maximum easily allowable altitude for your flight and submit the airspace authorization request online through DroneZone.
It is currently taking around a week or two for authorizations that fall within the UAS Facility Maps guidelines and longer if additional research is needed to get approval.  Authorizations are ONLY offered for airports not currently supporting LAANC as LAANC is the preferred means of processing.
One of the conditions of getting either a waiver or an airspace authorization is to provide details on how you intend to handle contact with ATC.  Contacting the tower via phone is the most general approach.  Some also monitor ATC frequencies so that ATC can more rapidly get ahold of them if needed.

Regulations have changed greatly since this answer was posted. There is now a requirement to get "prior authorization" before flying in most controlled airspace, regardless of whether the operations are conducted under Part 107 or under the Recreational Exception.. If I wanted to fly a radio-controlled plane in the Class D airspace of KIAB, for example, which is not currently supported by LAANC (hence the red color of the grid), it would not be sufficient to simply call the tower and notify them.  I would need to get prior authorization somehow.

@quietflyer is quite right that the rules for recreation have changed since the original post.  I don't have time to update everything yet, but recreational is supposed to be getting rolled in to LAANC or something similar.  I haven't kept completely up to date on that though as I fly under 107 which hasn't changed significantly since this post.

Its a bit simpler than that, the process is outlined at the FAA website for UAS; and is mentioned as requesting a waiver.
You have to fill out an online form which contains the time and date of operation; and submit it 90 days in advance (tick the box labeled 107.41 Operation in certain airspace, and then identify the aerodome and airspace class).
This handy PDF has some further guidance on the rules around UAS operations.
A final note that this applies only to the US.

Operators of an sUAS, or drone, should avoid flying near airports because of the difficulty aircraft pilots have in seeing or avoiding them. However, if you wish to operate your sUAS near airports in controlled airspace, you need to get airspace authorization prior to operation.
This used to be done by telephoning airport traffic control (ATC). Today, for most airports, operators can receive near real-time authorization to fly their drone under 400 feet in controlled airspace around airports.
It works through a system called the Low Altitude Authorization and Notification Capability (LAANC). It’s a collaboration between FAA and industry. LAANC “directly supports UAS integration into the airspace.” It provides drone pilots with access to controlled airspace below 400 feet and makes them aware of where they can and cannot fly.
LAANC automates your application and approval to fly in controlled airspace. Your request is checked against several airspace databases. If approved, the applicant receives authorization in near-real time. Even with this authorization, the pilot must still consider other factors such a weather conditions at flight time.
More information is available on the FAA’s LAANC website.

Welcome to AviationStackExchange. Is there anything you can add to your answer that my expound upon it more than AJ Henderson has done in his answer?

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text:
Can ATC assign speed adjustment to aircraft on final approach course?
According to FAA Order 7110.65AA,
5−7−1.b. Do not assign speed adjustment to aircraft:
4. Inside the final approach fix on final or a point 5 miles from the runway, whichever is closer to the runway.
However, according to ICAO Doc 4444,
4.6.3.6 Only minor speed adjustments not exceeding plus/minus 40 km/h (20 kt) IAS should be used for aircraft on intermediate and final approach.
I thought ATC can't assign speed adjustment if the aircraft is on final approach segment.(after having passed FAF)
However, according to ICAO Doc 4444, is it possible?
In real situation, what procedure does ATC usually apply?

While the visual separation and the visual approach is widely used in the US, there is less requirement for controllers to issue speed adjustment instructions,I think.

This will depends on jurisdiction. ICAO suggests a limit of 4 NM final. DOC 4444:

4.6.3.6 Only minor speed adjustments not exceeding plus/minus 40 km/h (20 kt) IAS should be used for aircraft on
intermediate and final approach.

4.6.3.7 Speed control should not be applied to aircraft after passing a point 7 km (4 NM) from the threshold on final
approach.

This is widely adopted, including by EASA. AMC2 to ATS.TR.210(a)(3) of EU 2017/373:

(b) The air traffic controller should use only minor speed adjustments not exceeding plus/minus 40
km/h (20 kt) IAS for aircraft on intermediate and final approach.

(c) The air traffic controller should not apply speed control to aircraft after passing a point 7 km (4
NM) from the threshold on final approach.

As usual, the FAA does things differently from everyone else. 7110.65AA 5-7-1:

b. Do not assign speed adjustment to aircraft:

(...)

Inside the final approach fix on final or a point 5 miles from the runway, whichever is closer to the runway

It is very common for air traffic control to apply speed control on final approach, as this is an essential way of ensuring efficient spacing between arriving aircraft at busy airports.

I guess perhaps US airports are more generously sized on average so they don't need it that much. At some moderately busy EU airports with just a single active runway to work with, it's entirely commonplace to interleave arrivals and departures, so you frequently hear TWR tell a plane first in line for landing "ABC, reduce to minimum approach speed, break break, XYZ, cleared for immediate takeoff" (as in, "line up and GTFO"). Often enough followed shortly afterwards by "ABC, go around, traffic on the runway".

"Frequently hear" No way. You make it sound like the average European air traffic controller (I happen to be one) is completely incompetent.

Sorry, but I never said or meant to imply anything like that. Actually, I'm truly amazed how the ATCs at these airports manage to consistently stuff 40-50 movements an hour onto a single runway. But I have personally witnessed two such go-arounds because the departing aircraft started it's takeoff roll a little later than was needed and didn't clear the runway in time (out of a sample of some 70? landings) so it does happen.

This is a place where the US is different from ICAO recommendations. You can see some of those differences at the FAA's website:

In the US, speed control is not to be assigned inside Final Approach Fix or 5 NM from runway end.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
Does the flight always switch to TRACON on a go-around in class B airspace?
I have been listening to class B go-around ATC recordings.  After the initial climb of the go-around, the tower gives the flight a heading and altitude.  Then, on some recordings I hear the tower say "contact departure" and a frequency.  But on others I don't hear that.
I can't tell whether:

It's optional for the tower to hand off the flight to departure in a
go-around and the tower can keep control the whole time until they
land, or

The tower always switches the flight to departure, but sometimes
that hand-off is left off of the recording.

Which is it?

They probably do because they have to push them out pretty far for sequencing. I don't think it's a requirement though, if the airport is quiet and they can just run the pattern I'm sure they can stay with tower.

If they stay on the tower frequency, is approach involved at all (even though they don't talk to the aircraft)?  Are they aware of the go-around?  Are they in touch with the tower about it?

It the go-around pattern's airspace permanently reserved for go-arounds only?

There is no "go around pattern" or associated airspace. There is simply the pattern, a racetrack with one of the long legs being the runway. Tower is responsible for this, so if they can stay in the pattern, they don't need to switch to approach. If there are a bunch of tightly sequenced arrivals though, they may need to go out, maybe as much as 25 miles, then approach and/or departure need to be involved.

@RonBeyer is right. If it's a clear night & no inbound traffic, the aircraft can simply fly a visual pattern with the tower. If it's IMC & vectors to an instrument approach will be needed, and/or this aircraft will have to be sequenced in with other arrivals, then they'll send him back to Approach. Either way, the Tower controller will probably tell the Approach controller (on a landline) what's going on.

It depends. As a rule of thumb, a missed approach will be handed over to approach (TRACON) to get a new approach clearance. Some places have local rules that allow approach to delegate this responsibility to the tower when it makes sense, which means the aircraft can then stay on the tower frequency for the new approach. This would normally require a few things, such as appropriate equipment being available in the tower (radar) and having a tower controller that is actually rated to be allowed to perform approach control.
In any case, when a go around is initiated, the tower will immediately call approach and coordinate what to do. One of the results of this coordination is the "heading and altitude" you have heard the tower give to some flights. Typically, approach will request to get the flight at a specific heading and altitude, and the tower will give those instructions before handing over the flight. Some airports will have very specific local regulations about which steps to follow, others allow the controllers to come up with a solution that fits the specific situation.

What heading and altitude does a pilot use while waiting for instructions?  Does he just join the pattern?

@JoelFan if they are on an IFR flight plan, the approach plate has missed approach instructions to use. If they are VFR, they'll be told what to do

One should distinguish between a “go around” (balked landing) and a “missed approach”. ATC’s response to each may differ.

@RonBeyer, I assume it's usually similar to a normal take-off in terms of heading and altitude?

@JoelFan not always. IFR missed approach procedures are pretty different from departure ones. VFR is the same until you turn crosswind to rejoin the pattern unless otherwise instructed.

@RonBeyer the ONLY time any controller would let an aircraft fly the published missed is if they asked for it for training purposes, or if there was no radar coverage. In the radar environment it is much easier to just vector the aircraft back for an approach, especially as the published missed can often include a turn back toward the arrival corridor and a hold over an approach fix.


text:
ADS-L Protocol implementation
I am trying to implement the ADS-L protocol in my application, but I did not fully understand it yet.
Data link layer (page 13):

Why CRC is at offset (Packet Length +1)Bytes if the CRC is included in the "Packet Length" of the first byte?

Network layer (page 16).

In the bullet points it is written that the network layer also contains a payload lenght indicator but the table below does not show any indicator.
Is the "(Packet Length - 3) * 8" the same length then in the table of the data link layer?

My interpretation would look like this:

What is my fault that the offsets do not match for me. Do I use the correct specification because on some pages issue 1 and on some pages issue 4 is written.

The protocol gets updated, it contained minor issues and in a future version they are fixed.


text:
What geographical area does a METAR cover?
According to the FAA's AC 00-45G section 3.1, a METAR is defined:

Aviation Routine Weather Report (METAR) is the primary observation code used in the U. S. to
  satisfy World Meteorological Organization (WMO) and International Civil Aviation Organization
  (ICAO) requirements for reporting surface meteorological data. A METAR report includes the
  airport identifier, time of observation, wind, visibility, runway visual range, present weather
  phenomena, sky conditions, temperature, dew point, and altimeter setting. Excluding the airport
  identifier and the time of observation, this information is collectively referred to as “the body of the
  report.” As an addition, coded and/or plain language information elaborating on data in “the body
  of the report” may be appended to the end of the METAR in a section coded as
“Remarks.” The contents of the “Remarks” section vary with the type of reporting station. The
  METAR may be abridged at some designated stations only including a few of the mentioned
  elements.

What doesn't appear to be defined is exactly what area the METAR covers. It's understood that a METAR is a reporting for a particular station's current conditions at a particular time, but is there a more discrete definition for where the METAR's conditions can be expected?
The definition of a TAF specifically states that the forecast conditions are expected in an area around the reporting station with a radius of 5 statute miles. The METAR has no such specification that I can find.

Geographically METARS are for a single specific location (the station identified in the report).
An automated METAR with no human supervision covers the conditions within approximately 6 inches of the weather station reporting it (for example, a badly positioned AWOS/ASOS station may be shielded from wind in some directions which affects what it reports).
A manual (or manually augmented) METAR covers the conditions the weather observer can physically see (clouds, precipitation, visibility, etc.) in addition to the ones they can measure (temperature, dew point, barometric pressure), which are valid for the point of measurement.

Practically speaking if the weather station is positioned well a METAR (like a TAF) is probably fairly representative of conditions within 5 miles, assuming there are no geographic features such as mountains which may mess with the weather. The data is still still "useful" within about 20 miles (e.g. KMTP uses the weather reported by KHTO, about 17 miles away), though you should obviously cross-check it - in that example the winds at Montauk (right on the water) can be noticeably different from the winds at East Hampton (further inland).
Some information is broadly useful over a wider area. FAR 91.121 requires you to set your altimeter to the value reported by a station "within 100 nautical miles" of your position when you're operating below 18,000 feet because things like barometric pressure tend to affect relatively large areas.

Since you mention it, there's a particular airport I fly into sometimes where they installed the ASOS gear right up against a tree line. You have to still fly over and check the sock because the winds it reports are often completely wrong. All the ASOS measure is which way the wind in the rotor by the tree line is going. Great answer, thanks.

@RyanBurnette There are actually guidelines on how to properly site a weather station - it gets screwed up amazingly often though.

AWOS/ASOS cloud cover isn't limited to the 6" cylinder of sky above the station - it has a much wider arc. That's how it calculates scattered, broken, etc.

It's been said before in another forum, that a report of rain combined with CAVU, means that the lawn sprinklers are hitting the automatic weather station.

@quietflyer there's a corn/soybean field at my airport near to the ASOS. Come harvest time we always have to edit the report, because the system thinks the visibility is very very low due to the (localized!) dust in the air.

@egid The ceilometer does just measure straight up. Automated SCT/BKN/etc are based off of time averages, not its view of the whole sky. One cloud parked over the AWOS all day will give OVC regardless of what the rest of the sky looks like.

A metar report is good within 5 NM of the station.
"VC" or vicinity of the airport is defined as between 5-10NM from the station.
"DSNT" or Distant is used beyond 10NM from the station.
http://www.ofcm.gov/fmh-1/pdf/FMH1.pdf page 42.


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How else can snowy/icy conditions disrupt airport operations?
In casey's fabulous answer here, he identifies several ways snow/ice can disrupt airport operations:

Continuous heavy snowfall can eventually close an airport if it
  becomes impossible to keep the runways clean, taxi routes become
  unusable due to drifting snow or snow transitions into sustained
  freezing rain (or runs out of de-ice fluid, yes IAH I'm looking at
  you). This is a transient problem and the closure would end as soon as
  conditions ease and the facilities can keep up with it.

Are there other ways that heavy snow (or ice) can disrupt airport operations?  Could it perhaps interfere with lighting or navaids?  Are there other airport functions that stop or slow in snow and ice?

Let's not forget low visibility from a snowstorm, potentially reducing the number of possible arrivals/departures

There are multiple ways by which snow can affect the operation in an airport.

Snow (and ice) in runway can affect takeoff by absorbing energy and can impact the aircraft after being kicked up by tires.
During landing, snow or slush in runway can reduce the deceleration rate by reducing friction coefficient, sometimes causing hydroplaning.
The runway length requirements in case of dry and wet conditions are different. This means that some aircraft cannot use the available runway in case of snow/wet conditions.
The requirements to clean the runways (and taxiways) continuously means that the flights will be delayed.
The snow may reduce the visibility of runway and terminal visual markers.
Ramp operations may be affected due to snow/ice.
Beyond a certain level of snow, the airports have to be closed as the aircraft cannot takeoff/land.
Also, it will be snowing in areas around the airport too. This means that there will be difficulty for people in reaching airport in the first place.

Also, see the FAA advisory circulars AC 150/5200-30C - Airport Winter Safety and Operations and AC 91-6A, Water, Slush, and Snow on the Runway

Airport workers often struggle to get to work.
This is generally OK for the first 12 hours or so but soon, people need to stop working and rest, especially those with safety critical roles which have legally mandated maximum working and rest periods.

Well, snow and ice would stick on aircraft, causing a dangerous situation because of unsteady airflow over lift surfaces. It would also decrease visibility, although it would not matter as much in an ILS except for seeing the runway at almost-touchdown altitudes. If the snow or ice is too much, it could interfere with the radar systems of the airport.

There are various ways which heavy snow or ice can disrupt airport operations. Visibility would certainly be an issue, as call tower operators would have trouble visually seeing aircraft and would be unable to accurately judge any distance between aircraft. Additionally, ground crews and pilots would certainly have a tough time trying to navigate around the airport.


text:
Can (part of) the fuel in your tanks be considered "property carried for compensation or hire"?
This question brings up that an experimental category aircraft cannot carry "persons or property for compensation or hire".  However, this raises a question regarding civil-registry refueling tankers (such as the venerable N707MQ).
Since the tanker is carrying fuel on behalf of another entity (the military unit they're contracted by to pass gas to) for compensation (contract payments), wouldn't the fuel that has been offloaded on-station be considered "property", backing N707MQ and its experimental (market survey) airworthiness certificate into a proverbial corner?
(Yes -- I know that contract tankers are considered Public Use aircraft when on-mission, which probably renders 91.319 moot, but it could easily be the case that a contract could say "operate IAW FARs with the exception of refueling operations", which'd inadvertently loop 91.319 back into the situtation.)

Surely it only becomes the property when it's transferred.  The fuel in a petrol pump only belongs to you once paid for.

@Simon -- is that still true for the propane in the delivery truck heading to your house that you already paid for at the start of winter?

Since the fuel in a refueling tanker is not, I believe, typically usable fuel I would suspect that it falls under the consideration of cargo.

@RonBeyer -- actually, most tankers are distributing fuel from their own supply (this is true for the Omega mods I'm pretty sure, and most military tankers as well)

Does the Air Force pay the contract, but the fuel only after delivery?

@Simon -- I believe that the fuel is paid for post-delivery (i.e. post-mission), but it's hard to tell as I'm not privy to any of their contractual language with their customers

This is an interesting question. It becomes more interesting if you can burn the fuel in the "transport" tanks in flight (if you planned your "fuel on board" figure accounting for ALL the fuel, and you can in fact burn it, then was it transported for compensation, or taken for flight purposes & then sold when you found you didn't need it?)

@voretaq7 Yeah -- which gets doubly interesting when you add ETOPS fuel planning to the mix (with having to plan for long driftdown or depressurized diversions and whatnot -- you may cut into your offload fuel for a divert/abort case like that depending on mission parameters)

N707MQ actually has a special type of experimental certificate, according to its registration it is a "Market Survey" aircraft as you note.
The FAA defines this as

Market surveys:  to conduct market surveys, sales demonstrations, and
  customer crew training for U.S. manufacturers of aircraft or engines.

This does not really provide us any special way to use the aircraft for commercial purpose but is worth noting.
Either way as you state 91.319 pretty clearly prohibits this. Interestingly enough 91.905 even prevents you from getting an FAA waiver to override 91.319 (as its not on the list of things you can have waved).
However 91.319 sub section H may provide us with a solution.

(h) The FAA may issue deviation authority providing relief from the
  provisions of paragraph (a) of this section for the purpose of
  conducting flight training. The FAA will issue this deviation
  authority as a letter of deviation authority.

As far as I know these tankers are primarily used in our own airspace for training missions which would fall under "conducting flight training".
This article is pretty interesting and covers a similar scenario for using experimental for commercial purposes.


text:
What's the rule of thumb for power and rpm in a constant speed prop?
I think I understand the relationship between the power lever and the RPM lever in an aircraft with a constant speed prop thanks to this question. However, what'd be a general rule of thumb for flying a small aircraft (e.g. Cessna 170) that has a variable-pitch propeller?
Edit:
I'm flying a Cessna 170 that has been modified to fit a constant speed prop. What I'm trying to determine are the general recommendations for flying with them. Things like: On takeoff you should use full power & full rpms, on cruise lower power to 75% but keep full  rpms, etc. I'm not sure if there are general recommendations to begin with.

The answer to your question is actually really simple: do not exceed the limitations specified in the POH.
Really, that's it!  Note that LIMITATIONS are not the same as RECOMMENDED PROCEDURES.  Recommended procedures are just that: recommendations.  Limitations (exclusively from the "limitations" section of the POH) are the hard and fast rules you must adhere to.
Full RPM in Cruise
As long as your POH does not impose a limitation on takeoff RPM (the Seneca II has such a limitation, for example), then there is nothing wrong with using max RPM from takeoff to shutdown. You won't damage the engine.  If the POH provides a limitation on engine RPM (specifically in Section II of the POH), you must adhere to that limitation.  It's common to see a 5 minute "max RPM" limitation on some turbocharged engines.  Great, takeoff at max RPM then pull the engines back to the top of the green arc until you land.  Easy!
Manifold Pressure & RPM
Mike Sowsun said:
"You should always follow the manufacturer's procedures but a general rule for direct drive normally aspirated engines is to keep the manifold pressure less than the RPM for cruise power. (eg. 24 inches of MP at 2400 RPM)."
If you decide to apply "rules of thumb" to all phases of flight you will be voluntarily handicapping your airplane's full potential.
Some definitions:
Oversquare - manifold pressure > RPM (23" @ 2100 RPM)
Undersquare - manifold pressure < RPM (23" @ 2500 RPM)
Square - manifold pressure = RPM (23" @ 2300 RPM)
Do not embrace the idea that manifold pressure should never, ever exceed the value of the first two digits of the engine RPM (sounds pretty arbitrary when I put it that way, right?).  It actually may be beneficial to run an engine "oversquare" in some cases!  This is another example of why knowing your aircraft's "Section II" limitations is important.  If 23" @ 2100 RPM falls within the limitations, go for it!!  You won't hurt the engine.  Same with 25" @ 2300 RPM or 26.756431" @ 2196.454634 RPM.  I think you get my drift.  As long as your manifold/RPM combination falls within the limitations set out in Section II of the POH, you're golden!!
Now, climbing out on a hot day with a garden variety Lycoming or Continental (like in 170)?  29" @ 2100 RPM probably isn't wise.  30" @ 2700 RPM?  Yeah, that's better!  25" @ 2500 RPM isn't bad but it's no better or worse that 26" @ 2700 RPM or 30" @ 2650 RPM.  Remember, there's nothing special about "square" manifold/RPM values.  I'd routinely run the IO-520 in my Bonanza at max manifold pressure and max RPM from takeoff to cruise and simply leaning to an EGT target as I climbed (and watching CHTs, of course).  Only when leveling off in cruise did I pull it back to an economical setting (sometimes oversquare, sometimes undersquare; always lean of peak).  The engine ran beautifully with good CHTs and incurred no unusual wear.  In fact, the engine's health improved.  Exhaust and plugs became noticeably cleaner and compressions improved.
In cruise, set the power to any combination of manifold/RPM you desire provided that setting falls within the limitations in your POH.  You won't hurt the engine with a valid manifold/RPM setting.  It's possible to put undue stress on a motor with a bad mixture setting, though.  POHs are notorious for giving poor guidance with regard to mixture settings.  Mixture settings require a little more attention above ~65% power but that's another topic entirely.
I recommend that everyone with engine questions read John Deakin's "Pelican Perch" articles.  He's THE source for fact based, accurate engine operation information. He happens to have touched on this "oversquare" myth here.  See Myth #2.
Edit: I credited John Deakin with my linked article.  I was mistaken.  The author of the linked article is Linda Pendelton.  She links to a Deakin article, though.
Mike Busch also addresses the oversquare myth here.

I accepted your answer because it's very complete. I wasn't aware it was possible to run your engine "oversquared" and I'll definitely check out the aircraft's limitations. I'm not sure if the Owner's Manual we have has a Section II (I believe POH's weren't mandatory at the time the aircraft was made).

The 170 manual I'm looking at has a limitations section (section IV).  You'll probably have to find the STC info to determine if the CS changes the original limits.

The general rule of thumb that I am familiar with is 2400RPM set by the governor control lever and 23in-Hg for manifold pressure set by the throttle lever.  This is what we do in a Diamond DA-40, for example.
I have seen the same settings used in other small aircraft, but I am sure there are variations between models.

I think acpilot covered most of it.  I'd like to add this little tidbit to assist in thinking about how to use your constant speed prop.
A constant speed prop is like a continuously variable transmission.   The main settings to be concerned with are climb and cruise.  In a climb scenario, particularly takeoff, it is desirable to have all power available, this is achieved by running your engine at a higher rpm.
In a cruise scenario, on the other hand, it is desirable to "upshift" by lowering the rpm at a cruise power setting.  This has the effect of opening the pitch of the propeller.  You may find additional efficiency at this setting.
I recommend, and it may exist in your checklist, to move the rpm all the way to the top end as part of your pre-landing checklist.  While the effect on your descent and landing is not of great concern, this simplifies the steps necessary in the event of a go-around.  You are already set up for max power, all that is necessary is to advance the throttle.

Thank you! Actually, my father is teaching me to fly that plane and that's exactly what he told me (and his father told him) about the prelanding procedure.

Most aircraft today either have a fixed pitch propeller, or have constant speed propellers where the RPM is controlled by a governor.
Some aircraft have ground adjustable propellers, or manually controllable, variable pitch propellers, but they are pretty rare these days.
Cessna 170's were all built with fixed pitch propellers.
It is possible to modify the C170 to a constant speed propellor but the engine would need to be changed, or modified, in order to supply oil pressure to the prop.
If you were to install a constant speed propeller in a C170 it would be operated just as any other light aircraft.
You should always follow the manufacturer's procedures but a general rule for direct drive normally aspirated engines is to keep the manifold pressure less than the RPM for cruise power. (eg. 24 inches of MP at 2400 RPM)

+1 for recommendation to follow the manufacturer's guidelines and square operating rules of thumb. Most engines can be operated "over square", within certain limits (most cruise-optimized fixed pitch props operate almost entirely over square during take-off, climb out, and cruise), but when in doubt, you're not likely to ever end up abusing the engine if you just stick to operating at square or below (i.e. MP less than or equal to RPM).

"While a constant speed propeller is technically also a variable pitch propeller,"* No they are NOT technically the same! A variable pitch propeller is missing the governor that a constant speed propeller has.

@jwzumwalt: A constant-speed propeller is a subtype of variable-pitch propeller (as the blade pitches of a constant-speed propeller are not fixed to a single value).  All constant-speed propellers are variable-pitch propellers, but not all variable-pitch propellers are constant-speed propellers.


text:
How precisely must ATC speed instructions be adhered to?
Is there any speed tolerance in regards of ATC speed control instructions during the landing approach?

You don't say what country, but I'll base this on the U.S... There is no "rule" here, however you can expected to be held to the Airmen Certification Standards. These standards are used by your designated pilot examiner (DPE) to determine if you are able to perform according to published standards.
For example, the Commercial Pilot ACS, depending on what phase of flight you are in, says that you should be able to maintain a specific speed +/- 5 knots. A lot of places in the ACS say "maintain manufacturers recommended speed".
So the FAA expects you to be able to reasonably maintain an assigned speed given that you were provided your license in accordance with ACS. That being said, if ATC assigns you a speed (N1234 Maintain 200kts until established), you should be OK if you are doing 190kts, or 210kts, but if you are blowing through there at 250kts, ATC will chime in and ask you to slow down.

I will add my experience that +/- 5 kts seems to be within tolerances. I have flown with pilots that have exceeded those limits, say +8-10 kts, and had our approach clearance cancelled with vectors out of the que. I don't think more than +5 is acceptable.


text:
Helicopter mast bumping in low g condition
The helicopter shown in the concrete pouring video seems to taking off from a hill and descending down in a way which can be classified as low G with low rotor loading. Although, the helicopter is three-bladed, mast bumping should be less of an issue, but from my understanding still worth considering.
Could a maneuver like this be executed in a similar way with a two-bladed helicopter such as a Bell 206 or Robinson. Furthermore, what is the reason for mast bumping being less of an issue for helicopters with more than two blades?

The famous video of the US Army seems to be describing exactly the condition as shown in the first video as potentially dangerous.

Re "The helicopter shown in the concrete pouring video seems to taking of from the hill and descending"-- my interpretation was that the helicopter was mainly climbing, followed by a slight descent and then a hover.

Oh, you meant after it dumped the concrete.  The question could use some clarification.  But a transition to a dive, even a rather steep dive, need not involve a reduction in G-load too much below 1 G as long as it is accomplished smoothly.

The answer is yes it could be used in that flight profile as the pilot of that AS300 was never in a low G condition with the rotor unloaded during those maneuvers.  Once he dropped the concrete, though he’s descending, he’s also accelerating at a high power setting, which is keeping the rotor loaded.  However, zero G mast bumping is still a risk that must be considered and thoroughly mitigated for during preflight for all helos with a semi rigid rotor system.
Mass bumping only occurs on helicopters with a semi rigid or teetering rotor system. It has to do with how a semi rigid rotor is supported by the mast in an under slung condition, where excess flapping can result in mast bumping.  It will generally occur in conditions like zero-G pushovers at low power settings or when attempting a steep slope landing.  Mast bumping is not possible on rigid, fully articulated, or elastomeric rotor systems where the rotor mounts cannot strike the mast, even with excess flapping.
Here is a pretty good recap on the subject of mast bumping.

Even helos with two bladed semi-rigid rotor systems can be flown through some pretty extreme maneuvers, provided the pilot flies within the design limits and does not unload the rotor.  I remember West Coast JetCopters, who provided the Bell 222 for the television series Airwolf, did some pretty extreme flying on those choppers and a 222 does use a teetering rotor system.  They just had really, really good pilots at the controls.  See 0:57 in the video below where the stunt pilots put a 222 through the helo equivalent of a hammerhead stall.

thank you very much, great answer. I did not fully consider that with the acceleration seen on the video the rotor stays loaded

I want a mod-only +500 vote for airwolf references!

I’m game for that!

Mast bumping is not an issue on articulated rotors (3 blades and up) because the system is not completely dependent on pendulum effect to keep the fuselage/rotor disc relationship within limits like with teetering systems.
And even with the disc completely unloaded, the articulated rotor still has some (although much degraded) roll authority due to the blade root offset from the center of the rigidly attached rotor hub, so that a pilot input to correct a roll induced by the tail rotor when zero G will limit the amount of roll.
Even if the rotor disc's displacement of the vertical axis did become excessive, the result is just the blade roots running into the flapping limits in their root attachments.  While it puts a lot of bending stress on the mast as the rolling moment bending is transferred through the mast and transmission mountings to the body, the system is designed to handle that and, it won't cause the mast to snap off like on the teetering rotor, where there is a concentrated bending load applied directly to the mast tube by the teeter stops.
So the AS-300 in the video, while it didn't go zero G in that departure, having a 3 blade elastomeric articulated rotor meant it wasn't in any risk of that kind of disaster even if it did go zero G.  Problem is, while you get some peace of mind when it comes to 0 G flight in your 3 blade Squirrel, now you have to worry about something the teetering pilot doesn't, ground resonance.

A follow up question: In the video of the US army it was often mentioned that the rotor can be reloaded by moving the cyclic aft. Is that true for any flight condition?

As long as you're moving, yes.  You can also pull pitch.  If you were stationary in some scenario where you were hovering at altitude and some crazy downdraft suddenly came down on top of you which cancelled out all the lift so you became a falling object, if it was enough to completely unloaded the rotor you'd be in the same situation and I think the only option would be to pull pitch.

Clearly you are speaking of what happens after the helicopter dumps the concrete.  A transition to a dive, even a rather steep dive, need not involve a reduction in G-load too much below 1 G as long as it is accomplished smoothly.


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What is a gold seal flight instructor?
What is a gold seal flight instructor and how does it differ from a usual flight instructor? Does it offer any additional privileges or prestige?

A gold seal flight instructor is obtained by having a ground instructor certificate and within the previous 24 months have a pass/fail rate of 80% or above and recommending at least 10 applicants.
It is a way for the FAA to recognize the best instructors in the industry.  You do not get any special privileges with the gold seal.
See AC 61-65 for more details.

No special privileges?  You do get to carry around that fancy gold seal.... http://i.imgur.com/ohFbydJ.png

You do get listed as such, which gets more students ringing your phone.

@KeithS speaking from experience, my gold seal did nothing to increase students calling, let alone showing up for flights. :)

If you are a DPE or a Chief tInstructor at 141 school you can substitute conducitng 20 practical tests in lieu of the 10 candidate/80% pass rate. You can also combine the requirements: (Two practical tests conducted equal the credit given for one applicant trained and recommended for a practical test.)


text:
Is there any country that requires window blinds to be open for takeoff and landing?
Does any country legally require the window blinds to be open for takeoff and landing? I'm asking about legal regulations that specifically mention window blinds, not airline procedures or general 'follow the crew instructions' regulations.
I only need one example, whether it's FAA, EASA, a national CAA or some other regulator, or an ICAO standard or recommendation.

welcome to aviation.SE. As seen previously, there seem to be no regulation and it really depends on the airline internal SOP. What makes you think that there is a regulation?

The airline's policy says if it is necessary or not. As far as I know, there is no regulation for this, and it is used according to the safet y procedures adopted by the company.

Very related: Why do we open the window shades during landing and take-off? (on skeptics.se)

I agree that the "potential dupe" is very related and addresses the situation. However, it only addresses FAA-land, while this question asks about other regulations. Perhaps if we reference the other question for FAA (as has been done), then edit out the FAA portion, this becomes a legitimate, non-dupe, question? It might have the exact same answer, but it does address the other regulatory bodies that way.

There is a regulation which indirectly covers it.  Certainly in the UK and I'm pretty sure in the US and EASA land, it is an offence to not comply with the lawful instruction of a member of the crew.  Therefore, if you are asked to raise the blinds, it is a legal requirement for you to do so.

@Simon: For sake of precision only, I assume you are not required to raise the blinds, you may help if you want (but you are not an employee of the airline, it may be against your religion, you may not understand, or your hearing aid may be off, or whatever hypocritical you can think of). I assume the offence comes if you lower the blinds when requested not to do it.

@mins.  Nope, it's exactly what it says.  Failing to comply with an instruction which is not itself illegal is an offence.  Rarely used but it has been.  Usually, the cabin crew will just mark you down as "idiot" and do it themselves.  But, if it is clear that you understand and that you are able to comply, you might be removed from the flight and prosecuted.  Generic laws like this, e.g. "dangerous driving" are there to provide for reasonable action when a specific law does not apply. Not raising the blinds when instructed is, quite simply, an offence in the UK.

@Simon: Thanks for the explanation. Interesting.

To add to what @Simon said, if it is in the Airline's operating procedures, they must also legally follow them (at least in the FAA world). I've seen many civil actions against airlines/employees for failure to follow procedure that had no other regulatory requirement.

@Simon that would make a good answer.

There is a regulation which indirectly covers it. Certainly in the UK and I'm pretty sure in the US and EASA land, it is an offence to not comply with the lawful instruction of a member of the crew. Therefore, if you are asked to raise the blinds, it is a legal requirement for you to do so.
Failing to comply with an instruction which is not itself illegal is an offence. Rarely used but it has been. Usually, the cabin crew will just mark you down as "idiot" and do it themselves. But, if it is clear that you understand and that you are able to comply, you might be removed from the flight and prosecuted. Generic laws like this, e.g. "dangerous driving" are there to provide for reasonable action when a specific law does not apply.
A valid defence would be to demonstrate that you were unable to comply to the extent that the prosecution is unable to convince a jury of your peers to find you guilty "beyond reasonable doubt".
So, in summary, albeit indirectly, it is a legal requirement for you to raise the blind when asked to do so in the UK.

"[I'm not asking about] airline procedures or general 'follow the crew instructions' regulations."

@J.Hougaard Ah, got it.  To be honest, I hadn't re-read the question after the edit.  Want me to delete?


text:
How do subjective experiences in the airliner cabin correlate with meteorological conditions?
There must be a better way of phrasing this, but: given the passenger's experience of riding through the air, what are the outside conditions that are responsible for that experience?
For example, if the passenger experiences a smooth, quiet ride (like driving on a smooth, newly-paved road), then the external conditions are perfectly still air.
What about the cases of:

a smooth but noisier passage through the air (like driving over a section of road with a different surface - not bumpier, just noisier)
consistent low-level but high-frequency vibration (like driving over a section of road with a regular pattern of deformation)
harder, occasional knocks and bumps (like driving over a road of variable quality)
hard, regular, sustained shaking (like driving fast over a rutted road)
repeated, irregular, harsh movements and lurches (like driving violently off-road)
a single, sudden jolt (like driving across the transition between road surfaces at different levels) - I've experienced this when crossing the transition between land and sea, and when intersecting the trail of another aircraft
a sudden very-high magnitude movement that comes out of the blue, of the kind that causes objects to fly around the cabin

To be clear, I am speaking of straight, level flight in clean configuration, in which different sound and vibration as experienced in the cabin are caused by conditions (pressure, movement, speed, turbulence, patterns of the air) outside - what are these conditions in each case?

noisier: You're siting aft of the engines.

@Jamiec That could be an explanation if the new noise were associated with your suddenly being moved out of business class into row 57, but not otherwise...

...or if the throttle is advanced ;)

@Jamiec In what sense is the operation of the throttle a meteorological condition, as specified in the question?

Are you asking about different 'types' of turbulence? If so, it's fairly close to this question or this one.

@Pondlife No, it's not about turbulence. Some items in the list may be turbulence-related, others not. Other causes could be related to conditions of air pressure, air movement, air speed and so on, and to changes in them).


text:
What is the relationship between the code of an Area Control Center (US) and associated Flight Information Region?
From the wiki page, what I understand is

an Area Control Center (ACC), also known as a Center (or in some cases, en-route, as opposed to TRACON control), is a facility responsible for controlling aircraft en route in a particular volume of airspace (a Flight Information Region). In the United States, such a Center is referred to as an Air Route Traffic Control Center (ARTCC).

My question: Is ARTCC code different from the ICAO-FIR code? Or are there anomalies?
For example, here's a NOTAM in the FAA site. AT the bottom, under the headline Other Information, the ARTCC code is given as ZDC. Can I safely assume that the ICAO-FIR code would be KZDC for that NOTAM?

Partial answer: ICAO members agree on FIR borders and which member has the responsibility of providing FIR services (information and alerting). FIR cover international (oceanic) areas too. Then ANSP (e.g. FAA in the US continental area, plus NY OCA) appointed by members provide services, which include information and alerting, but can, and most of time do, include more (e.g. separation). Often the FIR is split into airspace types (A, B, C..., but also UIR) to determine which services are provided. Apart East Coast, a single ARTCC handles a single FIR.

KZDC is the FIR associated with the ZDC ARTCC.  Think of KZDC as the airspace itself (the flight information region), and ZDC as the facility which services it.

Thanks. My question then becomes: If ZDC is the ARTCC code, will the related FIR code always be KZDC (In the United States)? Is there one to one mapping from ARTCC to FIR? Or Is it many-to-one/other way around?

Of the ARTCC's that have oceanic FIR's three use the same code for both, (ZAN/PAZA, ZHU/KZHU, ZMA/KZMA), but Oakland and New York centers each have two FIR codes. ZNY covers KZNY and KZWY (oceanic). ZOA covers KZOA and KZAK (oceanic).

Most are the same, but the scheme falls apart outside CONUS:

FIR
ARTCC
Name

KZAB
ZAB
Albuquerque

PAZA
ZAN
Anchorage

PAZN
ZAN
Anchorage Oceanic

KZTL
ZTL
Atlanta

KZBW
ZBW
Boston

KZAU
ZAU
Chicago

KZOB
ZOB
Cleveland

KZDV
ZDV
Denver

KZFW
ZFW
Fort Worth

PGZU
ZUA
Guam

PHZH
ZHN
Honolulu

KZHU
ZHU
Houston

KZID
ZID
Indianapolis

KZJX
ZJX
Jacksonville

KZKC
ZKC
Kansas City

KZLA
ZLA
Los Angeles

KZME
ZME
Memphis

KZMA
ZMA
Miami

KZMP
ZMP
Minneapolis

KZNY
ZNY
New York

KZWY
ZNY
New York Oceanic

KZOA
ZOA
Oakland

KZAK
ZOA
Oakland Oceanic

KZLC
ZLC
Salt Lake City

TJZS
ZSU
San Juan

KZSE
ZSE
Seattle

KZDC
ZDC
Washington


text:
What aircraft are parked in this naval air station?
In May 2020 I saw C-130 Hercules on a naval air station in Ventura County, California. Later, I decided to use Apple Maps so that I will remember what I have seen.
Apple Maps showed some C-130 Hercules, F-18 Hornets, P-3 Orions, and a few other aircraft. But when I zoomed into one of the air stations, I noticed some strange aircraft parked next to the P-3 Orions. They appear to be either a foreign aircraft, or some kind of experimental aircraft that I do not know.

Six of the unknown aircraft have a large swept wing, while two others have a delta wing featuring what looks like a canard.

It’s possible that these are adversarial aircraft from Draken. Not sure if the Navy has a contract with them or if they would be based out of point Mugu. You can see their inventory here http://www.drakenintl.com/catalog/aircraft-inventory

How come those are P3 Orions ?

@adityaguharoy You can identify the P3 Orion by the distinctive MAD (Magnetic Anomaly Detector) boom at the very rear.

They are private aircraft owned by ATAC for adversary training
https://en.m.wikipedia.org/wiki/Airborne_Tactical_Advantage_Company

Its main air operations base is at Naval Air Station Point Mugu, California

It operates Mk-58 Hawker Hunter, Israeli F-21 Kfir, A-4 Skyhawk, and L-39 Albatros II military aircraft in tactical flight training roles for the U.S. Navy, Air Force, and Air National Guard

Agreed. Hawker Hunters, owned by ATAC. On google maps, if you zoom into the Newport News/Williamsburg airport (PHF), you can see another 3 next to their hangar facility.

At the right end of the row of eight fighters, the two delta wings look like Dassault Mirages.  Trailing edge with a slight reverse sweep.  Ordnance just visible mid-span.  Upper fuselage flared towards the engine inlets.
The six fighters at left look very much like Hawker Hunters, but those are now flown only at air shows.  That guess led me to discover that they were stationed at Point Mugu AFB for a 2015 air show.
A video of the maybe-Hunter taxiing at Point Mugu in 2015 also
shows that one of the delta wings has registration N401AX,
which identifies it as a 1980 variant of the Dassault Mirage 5,
an Israel Aircraft Industries KFIR-C2.
Later, the video also shows the maybe-Hunter's registration, N330AX, confirming it as a 1959 Hawker Hunter Mk.58.

Right, a Mirage with a canard is a IAI Kfir. +1


text:
Determining the total drag of a propeller driven aircraft while turning with a load factor of n = 2?
I understand that the total drag includes the parasitic and induced component, with the parasitic component being the same as with straight and level flight.
How do I calculate the induced component?
The end goal is to produce a graph of drag versus airspeed.

I took the liberty of adding a clarifying sentence at the end based on content in your comments; feel free to delete if you think it detracts from the question.

Induced drag $D_i$ is proportional to the square of lift $L$:
$$D_i = \frac{2\cdot L^2}{\rho\cdot v^2\cdot\pi\cdot b^2\cdot\epsilon}$$
where $b$ is wingspan, $v$ is flight speed and $\rho$ is air density. $\epsilon$ is an efficiency factor which tends to be between 0.8 and 1. Compared to straight and level flight, induced drag will quadruple when turning at 2g and the same flight speed.
In order to know how much total drag goes up you need to know how much of your total drag is due to induced drag. Only then can the question be answered.
If your straight and level polar point is that for best range (when zero-lift and induced drag are equal), 2g flight at the same speed will increase total drag by a factor of 2.5.

Turning with a load factor of 2 is a steep turn of 60 degrees, so your entry speed will be faster to avoid an "accelerated" stall.  Vbg of a Cessna 172 is around 65 knots, but steep turns are performed at 90 knots.  The turn is entered by rolling, increasing angle of attack, and increasing throttle.
A practical way to determine increase in total drag would be to monitor needed rpm increase
(fuel flow) during the manuever.  One could also measure the actual AOA in the turn.
Theoretically,  the difference in AOA at level flight and AOA in the turn could be compared with the coefficient of lift and coefficient of drag polars for that airfoil for those respective angles (at the same speed) to derive a calculated solution.
Comparison with increased fuel flow in the turn should be close, but you may find the parasitic component increases too if the turn is not coordinated.

Do you know the L/D ratio at whatever angle-of-attack is being used in the turn?  If so, in a steady-state constant-altitude turn, Drag = (D/L) * Weight * Load Factor.
Note that the ratio of Lift / Drag is also the ratio of Lift coefficient / Drag coefficient.
Note also that the L/D ratio can easily be computed, if the Thrust or Drag for the same angle-of-attack is known for straight-and-level flight.
In a turn, the airspeed must be increased according to the square root of the increase in wing loading, to allow steady-state flight at the same angle-of-attack as we had in wings-level flight.
This answer treats the drag due to increased loading due to turning as being exactly equivalent to the drag due to increased loading that we would get simply by adding weight to the aircraft.  This is not exactly true, because in curving flight, the relative wind is actually curved, which has aerodynamic consequences.  But it is a very good first approximation for aircraft that have reasonably high flight speeds, and it is really the only practical way to tackle the problem short of an elaborate computer model.

for this particular question I do not have the Lift or Drag Coefficient. Is there any other way to calculate or a different formula for that D/L ratio?

I do know drag, but I am unsure how to compute the ratio, given that I do not know the lift

no we've been asked to plot drag against airspeed for different cases, nothing said about angle of attack, I guess I will write that the angle of attack is constant

does that mean that the drag to lift ratio is constant for both turning and level flight?


text:
What kind of analysis is done in front wheel plane?
I’m not sure how to describe this as a question… So here is an image:

What is the device? (marked in red)
What could the ground crew be analyzing near the wheel (marked in green)

They’ve been at it for quite a while

When you see several  people standing observing someone doing something like that, it's usually a training or demonstration session of some kind.  Nose gear servicing perhaps.


text:
What are the methods of reducing vortex?
I have tried reading the notes on the wing tip, but didn't understand well. How will the vortex be reduced on the wing tip.

Hi Abdulshakur, welcome to Aviation SE! Please explain in more detail what you have understood so far. It will be much easier to answer if you tell us where you got stuck. Did you look already at this answer?

Wing vortices  cannot be reduced. It is a byproduct of creating lift. The various wingtip shapes help to combine the airflow from the top and bottom of the wing and can make the wing act like it is longer and creating the same lift, but the vortex is still generated. The biggest vortices are created by the big airplanes when they are heavy, dirty (lots of flaps, etc. deployed) and slow (i.e. takeoff and landing).  Even my little 4-seat Cessna Cardinal creates vortices.
See Section 3.14 here
http://www.av8n.com/how/htm/airfoils.html#sec-vortices
of this great e-book "See How It Flies"
http://www.av8n.com/how/

Technically speed does not matter, angle of attack does. Higher AoA results in stronger wingtip vortices.

As a small plane pilot, I typically only encounter the larger planes near the ground when they are going slower. They can climb to altitudes well above me very quickly with their jet engines, so slow and dirty is where I am most concerned about being upset by their vortices.  I took off after a Gulfstream (G5 I think) on Saturday, had to wait 2-3 minutes after it departed before I was cleared for takeoff.

Yes, because when flying slow, planes generally have higher AoA. But they aren't the same thing. That's why I pointed it out. That is also why the vortices stop when the plane lands, AoA is 0, no vortices though the plane is still moving (slowly).

Wake turbulence is greatest when an aircraft is heavy, clean and slow. See this FAA paper

Huh, I would have thought dirty with flaps, slots, slats all hanging out for extra lift. I guess not tho: " The greatest vortex strength occurs when the generating aircraft is heavy-slow-clean since the turbulence from a “dirty” aircraft configuration hastens wake decay. "  That AC is waaay better than what was available when I got my license & Instrument rating many years ago.

Vortices are shed along the entire span of the wing and are proportional to the lift produced. To reduce the strength of the vortex at any single point along the wing, including the wingtip, you can increase the span. This does not change the required lift or the total amount of energy going into the sheet.


text:
Is interfering with the "go/no-go" decisionmaking of a flight crew a violation of 14 CFR 91.11?
ASRS Callback #216 tells the sordid tale of a pilot who was inappropriately intimidated by an armed guard at a public-use airport into performing an unsafe and illegal flight after landing there for an unplanned overnight stay and fuel stop:

I estimated that we had just under one hour of fuel remaining, and as it was getting dark and I was completely unfamiliar with the mountainous terrain, I decided not to fly on. I landed at ABC, a public-use airport. The only telephone available connected us to the lodge, which also operated the FBO. We told the desk clerk that we could simply camp overnight and buy fuel in the morning. The desk clerk said OK. We parked on a corner of the ramp, pitched a tent under the wing, and went to sleep.
Around midnight, we were awakened by an armed man in a "Special Police" uniform, who declared, "Leave immediately or I will arrest you for trespassing and you will spend the night in jail." We told him everything noted above, but he was adamant: leave or spend the night in jail.
I estimated that I had enough fuel to make it, although certainly not enough for legal night VFR flight. Faced with the alternative of going to jail, I decided to depart. It was a pitch black, moonless night. Although clear and with about 10 miles visibility, there were no lights visible and there was no discernible horizon. I climbed above the MOCA, flew on instruments for about 20 miles to the nearest airport, and landed. The FBO there pumped 21.6 gallons of fuel into my tanks, which have 22.5 gallons usable.
My mistake was in allowing myself to be forced, even by an armed guard, into making a flight which was against my better judgement, probably beyond my capabilities, unsafe, and illegal. It was only luck that it turned out all right.
I was able to reach the manager of the FBO later. He said that the "special police" had no authority to order a pilot to take off from the airport and that he would make certain the police knew this in the future so they would never do this to another pilot.

While this behavior from any sort of airport security personnel is beyond the pale and the FBO manager's response to hearing about it is absolutely correct -- the incident raises a burning question in light of 14 CFR 91.11:

No person may assault, threaten, intimidate, or interfere with a crewmember in the performance of the crewmember's duties aboard an aircraft being operated.

Is intimidating or threatening a pilot into taking off (vs staying put) considered a 91.11 violation by the thug, considering that the "go/no-go" decision is one of the most safety-critical decisions a pilot makes when preparing for a flight?

Violence  and threats should always be the first response. The Special Police officer does not need to know aviation law, because he has a gun.

From the quote in the question, it's not clear to me that the security guy was asking them to leave with the aircraft. Maybe just "camp outside the airport"?

What is "Special Police"?

@DJClayworth -- I'm not clear on how the "special police" badge got handed out (the story does not specify a location)

If the airport received (US) federal funding, then it must be available for emergency or distress landings. The guard could require the people not camp, but forcing a flight to an aircraft without sufficient fuel, reserves, WX briefing, etc. is unprecedented.
As a matter of practice, disoriented pilots who land at private and uncharted corporate airports in northern Canada are normally given fuel, food, and calls to get WX and file flight plans.
Perhaps someone better versed in Admiralty Law might comment.

I am not an aviation lawyer and I'm sure there are local implications from a property law standpoint that vary based on where in the country you are. However it is not a violation and the regulation clearly states why

§91.11   Prohibition on interference with crewmembers.
No person may assault, threaten, intimidate, or interfere with a
  crewmember in the performance of the crewmember's duties aboard an
  aircraft being operated.

You could go as far as to say that the oficier intimidated the pilot into breaking § 91.13 Careless or reckless operation. However in this case the intimidation occurred on the ground and outside the aircraft.

But "assault, threaten, intimidate, or interfere with a crewmember .. aboard an aircraft" or "performance of the crewmember's duties aboard an aircraft"? Seems that there are two interpretations. Unless it is a drone, how can a pilot fly without being "aboard"?

I guess it does depend where you stress the aboard the aircraft part and that would need to be decided by a court as there are clearly multiple interpretations.


text:
How does vertical separation work above FL400?
The information mentioned in "Jeppesen Airway Manual" is that after FL400, fight levels are FL430, FL470, FL510 for the 'even' FL's column.
Normally, I would expect every level divided by a thousand feet to be usable (FL410, 420, 430, 440 and so on). Why is this not the case above FL410?


text:
What do tower controllers do when the airport is quiet?
Listening Hayward Executive Tower on LiveATC in the night, I noticed ATC can be quiet for periods of more than 30 minutes, and sometimes the communication is not even with an aircraft, but with a maintenance car.
What do ATC operators when the airport is quiet?

I used to fly a lot of night freight back in the late 1980s and the 1990s, both in the U.S. and in Europe. While I can't answer your question, I can tell you that tower controllers always answered me promptly in the middle of the night (if the tower was shown as operating) when there was no other traffic but us. There was one exception; Valencia, Spain, where we would often land without clearance after repeated attempts to contact the tower.

Center controllers will happily "name that tune" or look up trivia to settle bets during dead hours, too.  It may be non-standard, but at least it keeps the pilots and ATC awake.

They monitor 121.5

Sometimes they take a nap. http://www.nbcwashington.com/news/local/Air-Traffic-Supervisor-Accused-of-Sleeping-in-the-Tower-118548444.html

When I used to fly at night and there was nobody else in the pattern they'd rate my landings.

I've heard that 123.45 can be a fun place to tune if you are flying and bored. Just make sure to only put it one one radio so you can be on either guard or atc if you are called.

The same things people do when working jobs like night shift guard shack operator... catch up on paperwork, read, study, listen to music, etc.

Can you provide a reference to this claim?

LOL! Sure, hanging out with my controller friends in the Marines. :) Do you have a reference to disprove it?

Listening to music with speakers is frowned upon in the cockpit, and by the same principal it should extent to controllers too. I'm not sure how that applies to military operators though.

"Listening to music with speakers is frowned upon in the cockpit..." Ah, no, sorry, it isn't. Simple proof of that is that most modern intercom systems, either panel-mount or stand-alone, have connections for external audio and/or things like iPods, and features like auto-mute when there is audio coming from a source like a VHF radio. Dynon and PS Engineering are just two examples of manufacturers that do that.

that's why I said "with speakers". The auto-mute function is specially designed for the use in cockpit. Question is, do controllers have equivalent audio equipment?

Speakers and headphones make no difference. Audio is audio. Auto-mute is available in both panel mount and standalone versions with speaker and headset outputs. The same technology is available to anyone, not just pilots and controllers. How long have you worked in avionics?

100% Agree with your answer. And I have done all of them at some point, including doing the Exams I am studying for and writing new procedures for the tower I am in. Music is allowed in a lot of towers, it is in the local instructions for some towers or in the case in the local ANSPs regulations or operations manuals. Speaker is safer then headphones in a tower as most towers don't use full headsets all the time. Some towers have a commercial radio going all the time.


text:
Why do most commercial solid-state Inertial Measurement Units (IMUs) measure angular rate and not angular acceleration?
The vast majority of MEMS-based IMUs are formed by an accelerometer and a gyroscope, providing linear acceleration and angular rate measurements respectively.
However, having available the angular acceleration would be useful for a more precise kinematic model when integrating to find the pose. Judging by this IEEE review, there are sensors for angular acceleration, therefore it doesn't seem to exist a fundamental limitation in the ability to have direct measurements of it.
Why is it that the standard has become to have angular rate as the source of information for rotational motion and not angular acceleration?

It is probably used internally to calculate the angular rate, what makes you think it's not? It may not be useful to other systems, but an IMU is an integrated piece of equipment that a manufacturer supplies. The raw data probably isn't useful outside the IMU.

My answer assumes that you're asking about having IMUs measure angular acceleration instead of angular rate; if meant they should measure angular acceleration in addition to angular rate, then I apologize for misunderstanding you.

However, having available the angular acceleration would be useful for a more precise kinematic model when integrating to find the pose.

I'm not sure where you're getting that information. When integrating to find the attitude, the angular rate is much more useful than the angular acceleration.
An attitude indicator essentially just takes the angular rate and integrates it. However, there are two sources of error here: errors from the sensor, and the fact that the direction of gravity changes as the earth rotates and as you move across the surface. But both of these errors can be corrected by mixing proper acceleration into the calculation.
On the other hand, if you only had access to angular acceleration data, you'd need to integrate twice to find the attitude: once to come up with an estimate of the angular rate and again to come up with an estimate of the attitude. (Why would you want to have to integrate to come up with an estimate of the angular rate, instead of just measuring the angular rate?)
This would introduce a third source of error into the attitude indicator: errors that result from accumulated errors in the angular rate estimate. I think these errors would be much more difficult to correct.
I think that the biggest problem is that in a coordinated turn, the angular acceleration is 0 and the lateral proper acceleration is also 0, so without measuring angular rate, an IMU would be unable to distinguish a coordinated turn from straight-and-level flight! That's a critical flaw.
In short, you need a reliable measurement of angular rate, and that can't be obtained from a measurement of angular acceleration.

You bring a good point with the coordinated turn, plus the errors derived from double integration (yes, there I had in my mind having both angular rate and angular acceleration, but is not what I stated). However similarly you could say that for an object moving with constant linear velocity: accelerometer can't measure it. I guess what calls my attention is the asymmetry on having in the IMUs velocity for rotational movement and acceleration for linear movement

Well, the asymmetry is because it's possible (and useful) for an IMU to measure absolute rotational velocity, whereas it's not possible (even though it would be useful) for an IMU to measure absolute linear velocity.


text:
What equipment is required to operate in Class "C" airspace?
I'm nearing my pre-solo test and I need to know this answer.

Which country or regulations are you asking about?

I don’t know.....What equipment do you need to operate in class Charlie airspace?  Where would you go to find that information?

A radio and a transponder.
As a pre-solo student, you really DO need to know this. If it is not familiar to you, I strongly recommend you read up on your airspace classes.